sounds like vacum leaks, i have this exact same engine only in a 150, when i went to replace a valve cover gasket it was coated in a penut butter colored cream, wet old oil, change the oil a few times clean as much as you can, and plug the vacum lines that are not conected, these engines are bullet proof, i dont think you have to many "major" problems, vacum leaks on these engines are the worst, especially with all the polution control stuff on our year of trucks, i pulled the egr valve off of mine and plugged i swear 100 hoses runs like a top, just get the hoses plugged and i bet it will run beter